Measurement methods are known, by means of wich variations in the concentrations of gases may be monitored, the measurements being continuously carried out in a laboratory. However, these measurements can only concern gases at atmospheric pressure or at lower pressures, while there is no difficulty in using any known type of apparatus for carrying out such measurements.
When, on the contrary, the purpose is to investigate very small variations in the concentrations of gases which may also carry along volatile products, while these gases present themselves at varying pressures, no means are available for continuously carrying out such measurements in the field over a long period. This is particularly so when investigating gases emanating from volcanoes, where the pressure of such gases may be considerable while fluctuating over a very wise range.
Devices are known which are capable of sampling gases from volcanic sources for measuring their concentrations by means of gas-phase chromatography, but these devices do not lend themselves to continuous measurements.
Also, while these devices have detection thresholds of about 50 ppm in the field and from 15 to 20 ppm in the laboratory, they are not sufficient for measurements which aim at predicting possible volcanic eruptions, since they are unable to detect either very small deviations in concentrations or the appearance of a new component appearing at a very low concentration. Now, this detection is indispensable for discovering and measuring gaseous components released through leaks from deep layers located for instance at some 30 kilometers below the surface, while these leaks may be affected by atmospheric and hydrological factors according to cycles, the evolution of which can only be determined by systematic continuous measurements over an extended period.
Briefly stated, methods are known for measuring with a high precision the variations in the concentrations of gases, for instance by means of a mass spectrometer, but then the measurements can only be made in a laboratory with bulky equipment, or otherwise methods are available for in-situ measurement of these variations, but then measurements are not continuous and are not precise enough for detecting small concentrations.